1. Field of the Invention
The present invention relates to a method for manufacturing three-dimensional physical structures, in response to computer output, using subtractive tool techniques in a layered fashion. This invention additionally contemplates a computer-aided apparatus that sequentially sculpts a plurality of layers of two dissimilar materials to construct a desired physical structure in a layer-by-layer manner.
2. Description of the Relevant Art
Traditionally, three-dimensional parts have been produced using subtractive machining methods. In such subtractive methods, material is cut away from a starting block of material to produce the desired physical structure. Examples of subtractive machine tool methods include milling, drilling, grinding, lathe cutting, flame cutting, and electric discharge machining. While these conventional machine tool methods are usually effective in producing the desired part, they are deficient in creating some complex geometries. Such methods are usually best suited for producing symmetrical parts and parts where only the exterior is machined. However, where a desired part is unusual in shape or has internal features, the machining becomes more difficult and often the part must be divided into segments requiring subsequent assembly. In many cases, a particular part configuration is not possible because of the limitations imposed upon the tool placement on the part. Thus, the size and configuration of the cutting tool do not permit access of the tool to produce the desired configuration. Additionally, a great deal of human judgement and expertise is typically required to execute conventional machining processes, making such processes relatively slow and expensive.
Various systems for three dimensional modeling have been proposed and/or developed to overcome the limitations inherent in conventional subtractive machining methods. For instance, U.S. Pat. No. 3,932,923 to DiMatteo contemplates the production of a plurality of individual planar elements, corresponding to thin cross sections of the object to be produced, responsive to signals generated from a contour follower. The planar elements are then stacked and physically joined together by various means to form the desired three-dimensional object. This technique has been known to be difficult to apply because an overwhelming number of planar elements may result from high resolution between layers, which may be required for non-uniform three-dimensional objects. The handling of these numerous elements, and the necessity that these elements be precisely stacked to be within tolerances, greatly lengthens production time.
Another method of three dimensional modeling is selective laser sintering, representative teachings of which are found in U.S. Pat. No. 4,863,538 to Deckard and in U.S. Pat. No. 4,938,816 to Beaman et al. That method contemplates the deposition of a powder, such as powdered plastic, in a bounded area to form a powdered layer. This layer, or a selected portion thereof, is then sintered by such means as a laser to bond the affected powder particles of that layer together, thus forming a discrete layer of the three-dimensional object. Successive alternating steps of powder deposition and sintering occur until the three-dimensional object is formed. Drawbacks associated with selective layer sintering include the fact that only a limited range of materials can be used and the inherent dangers presented by the production of toxic gases resulting from the reactions with the powder, coupled with a risk of explosion.
Alternatively, powder particles may be bonded together in a layer by use of a bonding agent, such as a ceramic. This process, known as three dimensional printing and developed at the Massachusetts Institute of Technology by Dr. Emanuel Sachs, is similar to selective laser sintering, except that it contemplates using a printer ink jet mechanism to deposit the bonding agent in a predetermined area of a powder layer, rather than using a laser to sinter the particles together.
Both powder-related techniques suffer from a drawback common to all other prior art three-dimensional forming techniques, with the exception of the aforementioned conventional machining. Namely, such processes are planar in nature, since the parts to be constructed are formed of discrete layers of material. Consequently, a large number of thin layers are required to form an object within given tolerances.
A three-dimensional object may also be formed by ballistic particle manufacturing, a technique taught in U.S. Pat. No. 4,665,492 to Masters. There, a first particle, denominated as an origination seed and constructed of material such as steel or a ceramic, is placed at the origin of a three-dimensional coordinate system. Working heads emit small particles or droplets of, for instance, a ceramic material, according to predetermined coordinates originating from the seed. These particles bond to the seed and to each other, whereby continued emission of droplets in the predetermined manner ultimately produces the three-dimensional object. While a wider array of materials can be used in this technique as opposed to other methods, ballistic particle manufacturing presents inherent tolerance problems because tolerance is a function of droplet size and droplet positioning accuracy, which is difficult to manage. Furthermore, the small droplet size (a droplet may be only a few microns in diameter) results in lengthy production time.
The most widely accepted commercial method of producing a three-dimensional object is known as stereolithography, taught in U.S. Pat. No. 4,575,330 to Hull and in U.S. Pat. No. 4,961,154 to Pomerantz et al. In this method, a bath of a photopolymer liquid is contained in a vessel. Generally, layer-by-layer solidification of predetermined areas of the liquid photopolymer surface is achieved through sequential exposure to a light source, such as a laser. Discrete layers, each newly-formed layer bonding to an immediately preceding layer, are formed until the desired three-dimensional object is produced. As each new layer solidifies, however, a shrinkage in its volume occurs, causing warpage, which leads to stresses in the formed part. These stresses may cause distortions in the part and thus lead to exceeding tolerances. While Pomerantz et al. disclose methods of compensating for the effects of shrinkage, such methods do not prevent shrinkage altogether. Additionally, stereolithography is limited to use of a photopolymer as the material from which the three-dimensional object is ultimately formed. Another disadvantage presented by stereolithography is that reactions with photopolymers frequently produce dangerous toxic gases.
A modified stereolithographic process is taught in U.S. Pat. No. 5,031,120 to Pomerantz et al. There, a photopolymer liquid is supplied only in discrete layers, and a supplied liquid layer, or selected portions thereof, is solidified throughout the entire thickness of the layer, differing from solidifying merely at the surface of a liquid bath, as taught in the aforementioned standard stereolithographic process. Any unsolidified liquid is removed from the layer, such as by vacuuming, and resultant voids in the solidified layer are filled in with a support material, such as wax. The support material is then allowed to solidify, after which time the entire newly-solidified layer is trimmed to a flat, uniform thickness by such means as a machining unit. After such trimming, subsequent layers are formed in like manner until the three-dimensional object is produced. This modified process presents a greater likelihood that an object thereby produced will meet tolerances, since no temporary support web is necessary, unlike the standard process, where such a web must be constructed for any overhangs of the part and then removed by hand. However, it is still subject to the same disadvantages associated with that standard process. Moreover, the machine required to implement the modified stereolithographic process is relatively complex and costly, and the rate of object construction is hindered by the additional steps required in this process.